(sous presse dans Biological Perspectives on Motivated Activities, R. Wong ed., Ablex: Northwood N. J., 1992)

WHAT IS SENSATION?

"gnoti se auton"

MICHEL CABANAC

Department of Physiology, Faculty of Medicine, Laval University, QUÉBEC, G1K 7P4 CANADA

          INTRODUCTION

"The importance of clearly defining (and redefining) terms that describe behavioral processes as our knowledge of the underlying physiological mechanisms advances has repeatedly been emphasized" (White 1989). This statement applied recently to Reward and Reinforcement (White 1989), and to Physiological Psychology (Milner & White, 1987) seems applicable to sensation.

Sensation has always been a matter of interest to philosophers. Plato (4th century BC) held that sensation and opinion are the main screens masking Truth, but the opposite view tended to dominate both before and after him. Heraclitos (5th century BC) taught that knowledge comes to man "through the door of the senses", and Protagoras (5th century BC) that the entire psychic life consists only of sensations. Aristotle (4th century BC), Plato's pupil, returned to the sophists' view that sensation is the gate of the soul. This notion can be traced through history up to nowadays. Hobbles (1651) wrote: "There is no conception in man's mind which hath not at first, totally or in parts, been begotten upon the organs of senses". Condillac (1754), taking the theoretical example of an inert statue, showed that the progressive attribution of the senses would allow the development of a complete mind in the statue. Thus he made it clear that the mind must use the senses to know and understand the world, and that the senses are necessary and sufficient to develop a mind. This notion was also accepted by Kant (1781) for whom, however, the senses were one of the two sources of human knowledge, the other one being understanding. "Sensationalism is the theory that all knowledge originates in sensations; that all cognitions even reflective ideas and ... intuition can be traced back to elementary sensations" (Titchener, 1909).

"All science, whatever the realm of application, has a common origin: the immediate experience of the observing person of the scientist himself" (Spence, 1948). The scientific process then proceeds in the sharing of evidence by two or more persons. Both processes take the sensory channels. The study of this obligatory channel is the origin of experimental psychology (Geldard, 1972).

The concept of the senses as portals of the mind has therefore turned to a commonplace statement among modern psychologists (Marks 1981). However Plato's image of sensation as a screen masking truth can be recognized as arising again from two problems: the difference between sensation and perception, and the complexity of attributes.

 

Sensation and Perception

Psychologists of the eighteenth century started to distinguish sensation from perception. For Reid (1785) a sensation occurs when an organ of sense is stimulated, and perception depends upon sensation but also includes a conception of the object perceived and an immediate and irresistible conviction of the object's actual existence. These definitions are still accepted nowadays: Levine and Shefner (1981) define clearly sensation as "the process of detecting a stimulus (or some aspect of it) in the environment", and perception as "the way in which we interpret the information gathered (and processed) by the senses". "In a word, we sense the presence of a stimulus, but we perceive what it is".

However, Schiffman (1982) considers as somewhat obsolete the differentiation between sensation and perception, yet brings a useful light to it: sensation is equated to physiology, and perception to psychology. This explains how two attitudes have developed until now.

For the first group, a stimulus associated with a context acquires a meaning (Titchener, 1909) and, to make perception still more complex, an adequate behavioral response to a stimulus carries a meaning and may modify perception (Tolman 1918).

The extreme of that tendency is gestalt perception which, by definition, incorporates sensation and content into a global experience (Koffka, 1939). Without going to such extremes many psychologists think that the distinction between sensory discrimination and perceptual discrimination is theoretical but not factual because it is not possible to dissociate the two elements (Pradines, 1928; Corso, 1967) and because bare sensation does not exist but rather is always included in complex perception (Merleau Ponty, 1945).

An explanation of this refusal to recognize sensation as a real entity might be found in the stimuli used to arouse perception. The psychologists concerned with perception refer almost exclusively to the auditory and visual perceptions (Banks, 1991). Hearing and sight are the main channels of communication; stimuli reaching the mind via these gates are therefore the most prone to bear a context-related message. Other sensory inputs, e. g. from the skin, are less complex and more amenable to be described as sensations.

The second attitude accepts the theoretical separating of sensation from perception. Huxley (1954) in his experiment with mescaline on himself, has reported that the drug modified his perception but that his sensations remained intact. Indeed, some psychologists (Geldard, 1972; Marks, 1974; Levine & Shefner, 1981) have no problem with the concept of sensation, perhaps because the stimuli they use, light, sound, temperature, pressure, chemical stimuli, are only slightly context-related, or not at all. Yet the refusal to recognize sensation as an entity, simpler and thus different from perception, pinpoints some weakness in the definition of both sensation and perception and justifies this essay. We shall return briefly to this point after having examined the problem of attributes. "A great deal of confusion would be avoided if psychologists at large recognized the fact that the sensation of experimental psychology is a simple, meaningless (or rather non-meaningful) process definable only by an enumeration of its attributes". (Titchener, 1909).

Attributes

"An attribute of sensation,... , is any aspect or moment or dimension of sensation which fulfills the two conditions of inseparability and independent variability" (Titchener, 1908). It follows from this definition that the attributes are always given when a sensation is given and that the nullifying of any attribute annihilates the sensation.

Wundt (1874) described sensation initially with two attributes: quality and intensity. He added affectivity in 1893 in the fourth edition of Physiologische Psychologie but then he withdrew it and returned to two attributes in 1896 (ref. in Titchener, 1908, p.345). Two attributes of sensation were added by KŸlpe (1893). To quality and intensity attributes of all five senses he added duration for the five senses and extension for vision and touch. Titchener (1908) added clearness to the list of attributes. Pleasure and pain were considered as plain sensations by J. Mill(1869) but affectivity was considered as an attribute of sensation by Ziehen (1924). On the other hand KŸlpe denied that pleasure-displeasure can be an attribute of sensation because it has attributes of its own (quality, intensity, duration) and because a sensation can exist without being pleasant nor unpleasant which contradicts the definition of attributes. For Yokoyama (1921), the hedonic tone is phenomenally distinct from the usually accepted sensory qualities. Beebe-Center (1932) considered affectivity as an attribute but not necessary attribute of sensation.

As is the case with pleasure, the other attributes also possess attributes of their own: vision has light and color (color has three attributes of its own: hue, brightness, and saturation); audition has pitch, and volumeness as proposed by Titchener (1908). The list of attributes therefore tends to branch on and on. This complexity however is not the main problem raised by the attributes of sensation . Boring (1942) listed four difficulties that contradict the theory of attributes.

                1) It is not clear that vision has the attribute of intensity.

                2) It is not possible to find an independent variability between visual brightness and visual intensity.

                3) Some sensations are too complex to fit within the rigid limit of the attributes.

                4) The question remains as to whether the attributes are not really separable.

Nowadays the psychophysicists are less concerned with the attributes of the sensations since they have developed methods of multidimensional scaling in which they devise a single psychological unit that can serve to measure simultaneously several or all of the attributes (Ennis & Mullen, 1986). This bypasses rather than suppresses the complexity of the attribute problems. One way to obtain such a result is to ask a subject how similar are two stimuli. Thus, as underlined by Marks (1974) "multidimensional scaling is concerned with psychological relations, the relations of various sensory (psychological) attributes or dimensions to each other".

In using the word attributes we have met a first difficulty; the list of attributes has grown longer, and is heterogeneous. There is need for clarification and simplification.

A Simpler Descriptive Model of Sensation

Rather than describing sensation as having attributes it seems more logical to me, from both the points of view of objective criteria and of sheer introspective evidence, to describe sensation in a mathematical way. Sensation can be described as a multidimensional space. Four dimensions define this space: quality, intensity, affectivity (fig. 1), and duration. Quality is a nominal dimension where elements appear as categories. Intensity, affectivity, and duration are parametric, quantitative dimensions. Attributes can take place within each of the first three dimensions of sensation represented by the X, Y, and Z axis of figure 1.

 

QUALITY OF SENSATION

It is neither obvious nor simple to explore the X axis of figure 1 and to list sensations. According to Aristotle animals and humans possess five senses (table I). This concept of sensation as originating from the five senses only has endured more than two millennia without being questioned much. The philosophers of the eighteenth century and the psychologists of the nineteenth have not added to the short list of the five senses, and modern textbooks on sensation study the five senses almost exclusively. However, sensation is not limited to the five senses and two trends have appeared in modern times.

The first trend can be considered as physiological, it consists in studying organs, the tongue, the skin, the inner ear, etc... rather than sensations. For example Levy-Valensi (1933) followed this trend in a treaty of physiology. Many psychologists adopt the same point of view (Geldard, 1972). Having described sensory organs, the physiologists more or less deliberately tend to incorporate them as source of sensation. Indeed, since they are not concerned with mental experience, the physiologists have identified and studied electrically and functionally numerous afferent pathways without making a clear difference between afferent path and sensation. Thus the list of sensations of table II, most commonly listed in modern textbooks (e.g. Wolfe, 1988), tends to increase beyond the five senses.

The other trend is the perceptual way to look at sensation with less concern for the sensory organs. This trend qualifies as psychological because it starts with an introspective step. Pain, on top of this approach, can be elicited from all parts of the body without a clearly identified receptor organ. Time (Boring, 1942; Schiffman, 1982), orientation (Schiffman 1982), kinesthesia (Corso, 1967; Schiffman, 1982; Geldard, 1972; Ludel, 1978), perception of space (Schiffman, 1982), organic sensation (visceral, hunger, thirst) (Geldard, 1972) have thus been proposed as sensations in addition to the five senses. However, the common feature of these perceptions is precisely to be vague and to pertain to perception as defined above rather than to sensation sensu stricto.

It is possible to reconcile both trends, by following the methods of physiology as well as introspection as advocated by Titchener (1909) and Strauss (1963). This process must take a semantic first step (fig. 2) to differentiate sensation from sensitivity.

In the following we shall call sensitivity the capacity of an afferent neuron to detect a physical or chemical change occurring at its endings and to transmit this information to the nervous centers. The sensor part of the neuron can sometimes be identified histologically with a specific organ, sometimes as a free nerve ending, and sometimes has not been histologically identified. The afferent pathway can be constituted of only one neuron but most of the times contains a chain of neurons. Sensation can be defined neither in terms of chemistry or physics, nor in terms of neuron function, but as a cognition. The brain possesses properties that can no more be explained by its neuronal constituents than life can be explained by the atomic or molecular properties of the constituents of the living cell (Bunge, 1989). Consciousness is one such property and sensation can be defined as the emergence of sensitivity into consciousness (fig. 2).

This definition rules out the use of "sensation" and "sensory" in the absence of a nervous substratum such as in Bacteria (Miller et al, 1989) or in the immunology system (Deschaux, 1988), and when no such substratum has been described as is the case with time. On the other hand, it is in conformity with the definitions found both in the Webster and the Oxford English dictionaries. Both dictionaries define sensation as the mental process due to stimulation of a sense organ. The next problem will then be reduced to identifying the sensor organs.

An important inference to be drawn from this definition is that sensation is not limited to the five senses. We know, from clinical evidence that pain can be felt from any locus in the body with a small number of exceptions: skin on the center of the cheek and on the olecranon, and the nervous tissue of the brain. Pain is not the sole sensation aroused from inside the body. Mere introspection tells me that, if I close my attention to the outside world and concentrate on my own body, I can feel slight visceral sensation from my limbs, trunk, chest, neck, and head. During disease, pains can be aroused in any part of the body but in addition during health I can also feel vague and obscure messages from my viscera, heart-beat, throbbing, movements in the abdomen, and of course, hunger pangs and need to urinate or defecate. Therefore, it may be stated as a new postulate that any of the various afferent pathways discovered by the physiologists is potentially a source of sensation.

The above hypothesis regarding the origin of sensations calls for four remarks.

1) The sensory window open to the outside world is limited to less than the short list of table II. Undoubtedly, it would be useful to be able to detect and sense more than these variables. For example, we could make use of a sensitivity for speed rather than acceleration (included in Wolfe's spatial sensation), to perceive ionizing radiations, to be able to analyze more completely the air and the water in our environment and foods, and finally to sense time, if time exists. Yet the existing spectrum of sensitivities and sensations has proven efficacious enough to ensure the survival of the species up to now, but the understanding of the universe is not facilitated by the limited information sensed.

2) In addition to information on the outside world, the brain receives also a vast amount of information about the physiological state of the milieu interieur. Table III lists the nervous sensors identified so far, that sense physical and chemical variables from within the body. Most of these afferent pathways are limited to a bundle of only a few neurons, often C fibers. The contrast between this meager input and the large avenues of the classical senses may explain how the latter have dominated psychology. A whole vagus nerve collecting mechanical, chemical, thermal, and pain information from all the viscera contains no more than 40 000 afferent fibers, whereas olfactory sensation collects information from over a hundred million fibers, vision from 1.2 million fibers in the optic nerve. One cochlea alone contains 30 000 afferent myelinated fibers. In addition a qualitative step, that would increase the relative importance of the five senses, may exist between large, fast, and frequent spikes in A fibers and small, slow, and infrequent spikes in C fibers. It is not surprising, therefore, that the sensations in our viscera are only episodic and imprecise, as compared to the five senses, and have been long disregarded as sensations.

3) The quality of a sensation is determined by anatomy, but the pattern of action potentials within an afferent pathway might also account for this dimension.

4) The identification of a sensation is likely to depend on the existence of a semantic support as a conceptual medium. The short list of tastes, sweet, salt, bitter, and sour has been extended recently by adding one sensation: umami (Yamaguchi, 1987). When no word is available to describe a sensation probably we tend to ignore this sensation.

Figure 2, that sketches the above definition of sensation as a sensitivity which emerges into consciousness, can be completed now to incorporate perception (Fig. 3) by adding multiple simultaneous sensory inputs and, overall, a cognitive input from memory.

 

INTENSITY OF SENSATION

Another advantage of defining sensation as the emergence of a sensitivity into consciousness is to relate sensation to the stimulus in two steps, via physiology: From stimulus to action potential, from action potential to sensation. If a sensation depends on the action potentials aroused by a stimulus, then the intensity of sensation is related to the frequency of spikes arriving at the centers (Adrian, 1928). In turn this total number of spikes depends on the individual frequency in each afferent fiber and on the spatial summation in the group of activated afferent fibers.

The rate of discharge of many peripheral sensors is time dependent or, to use the physiological terminology, adapt quickly. In other words, their firing frequency is a derivative of the time:

Firing rate = dstimulus/dtime

This property of the sensor is transferred to sensation. Thus, the sensory modalities which are equipped with such a quick adaptation become rate-sensitive, such as olfaction. This amplifies the sensation of directional gradients, and permits anticipating regulatory responses.

The above physiological definition of intensity solves some problems aroused in sensory modalities where the borderline between quality and intensity is not easily defined on psychological grounds only; e. g. the controversy about the intensity of white, grey, and black (Boring, 1942, p.132). There are cases where ambiguities have developed, probably in relation with inadequate semantics. The best known case is that of Newton (1704) who equated brightness and intensity until Helmoltz (1860) straightened the case. Yet, the explanation of the intensity of sensation by the density of action potentials remains largely theoretical because physiology has not accumulated enough knowledge to bridge the gaps between stimuli and sensations. On the contrary new knowledge tends to make the understanding of the magnitude of sensation more difficult. Modern physiology has confirmed the existence of reafferent loops within the central nervous system, and of efferent innervation of some sensors.

The discipline of psychophysics, a word coined by Fechner (1860), is devoted to the quantitative measurement of the relations between stimuli and sensations. Fechner measured the sensations of the senses (sinnliche Empfindungen) using sensation with the usual meanings of this word. He was mainly interested in matching sensation with the actual intensity of the physical stimuli presented to the subjects. Psychophysics is interested mainly in measuring the threshold and the magnitude of a sensation in response to a stimulus. To treat psychophysics in one chapter would be out of place and interested readers should satisfy their curiosity in the reading of specialized textbooks (Galanter, 1962; Manning & nstock, 1968; Marks, 1974; Moskowitz, 1974; Stevens, 1975; Atkinson et al., 1988) and reviews on the senses, olfaction (Cain, 1988), taste (Bartoshuk, 1988),and audition (Green, 1988).

 

AFFECTIVITY OF SENSATION

Pleasure is the state of mind aroused by a pleasant cause. Titchener (1908) took great efforts to refute the hypothesis that pleasure is a peripheral sensation. This refutation entails that pleasure may reside in any kind of mental experience. As stated by Nafe (1924): "all sensory modalities, with the possible exception of pain, involve both pleasant and unpleasant qualities". Duncker (1940-1941) listed four types of causes of pleasure: i) sensory enjoyment (or displeasure) i.e. to enjoy the stimulus or the consequences of behavior; ii) aesthetic enjoyment, i.e. strive for a better understanding; iii) desire (for a steak, a book, a love, etc), not a reaction but the fulfillment of a need, and iv) pleasure in achievement, dynamic joy of succeeding, in victory. Nafe's statement as well as Dunker's first category justify the description of sensation as being tridimensional (fig. 1).

The adjective pleasant qualifies a cause which arouses pleasure. Pleasantness describes the quality of a pleasant cause. It is possible to share with others the pleasantness of a cause, but we are never sure that they share the pleasure aroused. The antonyms of pleasure, pleasant, and pleasantness are displeasure, unpleasant, and unpleasantness. Pleasure and displeasure merge into indifference then into one another along an affective gradient (Young, 1959) from extremely negative (distress), to extremely positive (delight). Affective and affectivity apply to the realm of pleasure and displeasure. Following Beebe-Center (1932), "hedonic" has tended to replace "affective" in the literature, but these adjectives are synonymous and, in my opinion, affective is better suited because the Greek root, "hedone" means pleasure only. There is no place for displeasure under the word hedonic, yet the avoidance of displeasure is pleasurable and is therefore similar to the seeking of pleasure.

The affective process was defined by Wundt (1902) and he proposed three dimensions to the affective process: pleasantness-unpleasantness, excitement-inhibition, and tension-relaxation. However, Titchener (1908) showed that judgements of pleasantness and unpleasantness are direct, easy, and natural. Young (1959) defines the affective process from its three attributes: sign because the experience can provide pleasure or displeasure, intensity, and duration. Yet these attributes are not necessary when we define sensation itself as tridimensional (plus duration).

Sensory Pleasure and Behavior

In the commerce of a subject with incentives, it has been shown experimentally that the wisdom of the body leads the organism to seek pleasure and avoid displeasure, and thus achieve behaviors which are beneficial to the subject's physiology (Cabanac, 1971). Relations exist between pleasure and usefulness, and between displeasure and harm or danger. For example, when subjects are invited to report verbally, the pleasure aroused by a skin thermal stimulus can be predicted knowing deep body temperature (Cabanac et al., 1972; Attia, 1984). A hypothermic subject will report pleasure when stimulated with moderate heat, and displeasure with cold. The opposite takes place in a hyperthermic subject. Pleasure is actually observable only in transient states, when the stimulus helps the subject to return to normothermia. As soon as the subject returns to normothermia, all stimuli lose their strong pleasure component and tend to become indifferent. Sensory pleasure and displeasure thus appear especially suited to being a good guide for thermoregulatory behavior.

The case of pleasure aroused by eating shows an identical pattern. A given alimentary flavor is described as pleasant during hunger and becomes unpleasant or indifferent during satiety. Measurement of human ingestive behavior confirms the above relationship of behavior with pleasure. It has been repeatedly demonstrated in the case of food intake (Fantino, 1984), that human subjects tend to consume foods that they report to be pleasant and to avoid those that they report to be unpleasant. Pleasure also shows a quantitative influence: the amount of pleasurable food eaten is a function of alimentary restrictions, and increases after dieting. The result is that pleasure scales can be used to judge the acceptability of food.

Thus, in the cases of temperature and taste, the affective dimension of sensation depends directly on the usefulness of the stimulus to the subject. This was already noticed by Aristotle (quoted by Pfaffmann, 1982). The word "alliesthesia" (Cabanac, 1971) was coined to describe the fact that in response to a given stimulus a sensation is not necessarily constant but can move on the Z axis of figure 1. The affective dimension of sensation is contingent. Alliesthesia underlines the importance of this contingency in relation to behavior: A given stimulus will arouse either pleasure or displeasure according to the internal state of the stimulated subject. The seeking of pleasure and the avoidance of displeasure lead to behaviors with useful homeostatic consequences. The behavior of subjects instructed to seek their most pleasurable skin temperature could be described and predicted from their body temperatures and the equations describing their behavior were practically the same as those describing autonomic responses such as shivering and sweating (Cabanac et al., 1972; Bleichert et al., 1973; Marks and Gonzalez, 1974; Attia and Engel, 1981).

It is possible from verbal reports to dissociate pleasure from behavior and to show thus that the seeking of sensory pleasure and the avoidance of sensory displeasure lead to behaviors with beneficial homeostatic consequences. Pleasure therefore indicates a useful stimulus and simultaneously motivates the subject to approach the stimulus. Pleasure serves both to reward behavior and to provide the motivation for eliciting behavior that optimizes physiological processes. One great advantage of this mechanism is that it does not take rationality nor a high level of cognition to produce a behavior adapted to biological goals. As soon as a stimulus is discriminated, the affective dimension of the sensation aroused tells the subject, animal or human alike, that the stimulus should be sought, avoided, or ignored.

Experimental evidence in the narrow field of sensory pleasure thus confirms the Epicurean general principle which states that pleasure and displeasure are linked to the well-being of the organism (Lehman, 1914) and according to which: "when it helps and encourages (the vital movement) it is called pleasure, satisfaction, well-being, which is nothing real but a movement in the heart" (Hobbes,1651).

Pleasure as Seen by Philosophers and Psychologists

The relation of pleasure to behavior was regarded as obvious by the Greek philosophers Aristotle (284-322, BC) and Epicures (241-170, BC, see Conche, 1977): "Life and pleasure, as we can see now, are not separable; for without behavior there is no pleasure, and pleasure improves behavior" (Aristotle).

After the Greeks, many philosophers and thinkers such as St Augustine (354-430), Montaigne (1533-1592), Gassendi (1592-1655, see Bloch, 1971), or Sulzer (1751) recognized in the affective experience a great role, if not the essential role, as a motivation. Bentham (1742-1832, see Bowring , 1838) based his "greatest happiness principle" on pleasure, "the spring of action". Kant (1788) and J. S. Mill (1863) were more concise but equally clear on this point. For Freud (1920) the "pleasure principle" determines the aim of life. An important analysis of the role of the affective process in behavior has been carried out by Duncker (1940-1941): "A search for the ultimate motives of human conduct cannot disregard pleasure which many eminent minds have considered to be the fundamental motive, or at least an important one. Others, to be sure, have held that pleasure is the outcome rather than the motive or goal of human striving... There cannot be the slightest doubt that many human strivings bear some kind of reference to pleasure, and likewise that many pleasures bear some reference to striving."

It is therefore generally assumed that a motivated behavior is oriented by the incentives received by the subjects (Killeen, 1962; Nutin, 1975; Toates, 1986). "From every point of view the affective processes must be regarded as motivational in nature" (Young, 1959). Yet pleasure has never been popular in history for moral reasons. In addition the excesses of psychoanalysis have led to the rejection of all mentalistic explanations of behavior. As a result pleasure is shunned by most recent textbooks of psychology and even of philosophy, a drawback of behaviorism in modern literature. According to the part they attribute to pleasure, Duncker (1940-1941) has sorted philosophers into two schools: "Hedonists" for whom pleasure is the fundamental motive, as opposed to "Hormists" for whom pleasure is the outcome. "Hormism" was coined from McDougall's "hormic force" (1923) (a force that urges us to strive). One can easily recognize in hedonism and hormism the ancient opposition of the philosophers of the Garden, the Epicurists, and those of the Portico, the Stoicists, or with Pl_ (1982) the morals of pleasure, as opposed to the morals of duty. This opposition has lasted through the centuries. It is interesting to notice that this dual way of looking at pleasure can be found as an internal fracture within most societies, families of thought, and churches. Eventually duty always wins its struggle against pleasure. One good reason for the rejection of pleasure from the realm of science was the lack of experimental evidence and the fact that the philosophers' conclusions were based on their own introspection only. This rejection may be based on earlier approaches to these phenomena but, "as new techniques are developed, our ideas often have to be revised to encompass the new informations obtained" (Teitelbaum, 1964). The analysis of the cause for the dominant rejection of pleasure is out of place here. Interested readers will find it together with a historical review of epicurism in Pl_ (1982). Additional documentation on the relation of pleasure to behavior will be found in Toates (1986) and in Lea et al., (1987).

 

CONCLUSIONS

Once accepted the cogito ergo sum, the whole scientific knowledge bears on two postulates:

                             a) a world exists around me, and

                             b) I can exchange evidence with others.

Both of these postulales need the channel of sensation. Therefore our knowledge of the world, including ourselves is filtered twice. Once by the narrow chemico-physical window of the senses, and once by the biological and cultural format of our brain. The way we see sensation might have, therefore, some repercussion on the way we think.

The first hypothesis proposed here, according to which any afferent fiber is susceptible to arousing a sensation, presents several advantages. First the theory is simple. Then all the various categories of sensations are lumped into one single class whereas classical categorization listed many different sorts of sensations with different attributes. Thus is suggested a fundamental unity of the sensory input to the central nervous system.

The second hypothesis presented here regarding the structure of sensation can be examined from the points of view of both phylogeny and ontogeny. This chapter was devoted to human sensation but we may step back a little in phylogeny and try to guess about the origin of sensation. Medicus (1987) has reflected on the process from a behavioral point of view. Sensation emerged from a purely reflective behavior. A Darwinian approach tells us that sensory messages became conscious when this emergence proved useful to the organisms that first acquired it. To be useful, sensations needed to describe the quality, the intensity, and above all the usefulness of environmental stimuli; therefore, it is likely that sensation was immediately multidimensional, as defined above. Thus sensation gave decisional advantages to the first animals which possessed it, by freeing them from the need for an infinitely complex hardware reflex network in their nervous system.

Finally, if, as we saw in Introduction above, sensation as the gate to the soul is commonplace for psychologists and philosophers, it remains that the structure of sensation has an important consequence. If sensation is the phylogenic and ontogenic origin of the conscious experience, then any conscious event is likely to bear fundamentally the same structure. Indeed, introspection tells me that this is the case, and that any conscious feeling has quality, intensity, affectivity, and duration.

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TABLE I

ARISTOTLE'S FIVE SENSES

            EyeEarNoseTongueSkin

 

TABLE II

BEYOND THE FIVE SENSES

SensationSensor, organVisionretina, eyeAuditioncochlea, inner earGustationgustatory buds, tongueOlfactionolfactory neurons, C fibers, nasal mucosaThermalnerve free endings, dermTactilenerve endings, hair and Paccini's Meisner's and Krause's corpuscles, dermSpatiallabyrinth, inner ear; muscle spindles, muscles; Golgi endings, tendonsAlgicfree ending C fibers, everywhereAfter J.M. Wolfe (1988) in Readings from the Encyclopedia of Neurosciences: Sensory systems II, BirkhŠuser, Boston.

TABLE III

LIST OF THE NERVOUS SENSORS WITHIN THE ORGANISM

__________________________________________________________

Sensor
LocationSensed variable
MechanoreceptorsBig arteries:
        sino-aortic
        cardiac
Pulmonary
Venous
Digestive tract
Muscular
Tendinous and articular
blood pressure
cardiac repletion
    (blood volume)
pulmonary tension, volume
venous volume
digestive tube repletion
muscle length
tensionChemoreceptorsBig arteries:
Glomus, aorta, carotid art.
Digestive tract
Medulla
Hypothalamus
Liverblood Pa02
content of digestive tract
blood PaC02, pH
blood hormones
portal glucose
ThermoreceptorsHypothalamus
Spinal cord
Digestive tract
local temperature
OsmoreceptorsHypothalamus
Liverosmotic pressure
osmotic pressure

 

FIGURE CAPTIONS

Figure 1 Theoretical model of sensation as a mutidimensional experience in response to a stimulus. The fourth dimension, duration, cannot be represented graphically. Quality describes the nature of the stimulus, a non parametric dimension. Quantity describes the intensity of the stimulus. Affectivity, positive (pleasure), negative (displeasure), or nil (indifference) describes the usefulness of the stimulus. Pleasure can be modified by the subject's internal state or his past history.

Figure 2 Theoretical representation of the elements of sensation. Sensitivity is the set of elements composed of a specialized sensory organ, messages to the central nervous system, and afferent pathway. If the message reaches consciousness, an emerging property of the central nervous system, then sensitivity becomes sensation.

Figure 3 Consciousness is not limited to sensation. Perception above sensation incorporates the sensory messages with information from other sensory modalities and also with information stored in memory.